The invention relates to a method for synchronizing data transmitted between at least one transmit terminal and a receive terminal via a transmission channel with unmanaged latency. It also relates to a data transmission system to carry out such a method.
The invention enables notably the synchronization of the signals originating from a sensor or a set of sensors connected to a central unit via wireless links. It applies notably to the acquisition of biological, and more particularly neuronal, signals.
Biological signal acquisition systems have existed for a long time, but it is only recently that miniaturization has enabled the development of wireless acquisition systems. Conventional wired systems, such as EEG measuring caps, for example, physically connect the sensor(s) close to or in contact with a person or an animal (“the patient”) to measurement electronics, which are in turn connected by cable to a data processing unit, which may notably be a computer. Conversely, wireless systems integrate the acquisition electronics at the location of the measurement and transmit the data to the data processing unit via a wireless communication means.
During the acquisition of biological signals, one of the main subjects of study is the influence of external parameters on the measured quantities. In the neurosciences, for example, attempts are made to detect variations in the electroencephalography (EEG) signals in response to a sensory stimulus (flash of light, calling of first name, etc.). To do this, it is necessary to synchronize the time of the sensory stimulus and the different EEG signals, i.e. to place them on a common time base.
Synchronization of the data does not pose any particular difficulty in the case of conventional systems, in which the signal acquisition sensors are connected to the data processing unit in a wired manner. It suffices in fact to use a channel of the acquisition system as the input for the stimulus. For example, the signal activating a flash can be redirected onto one of the recording channels of the acquisition system. An acquisition system of this type is shown in FIG. 1, where the reference P indicates a patient undergoing an EEG examination (only his brain is shown), DSS indicates a sensory stimuli generation device (for example a flash, a screen, a loudspeaker, etc.), SA indicates a data acquisition system, UT indicates a computer serving to control the device DSS and also to process the EEG signals acquired by the system SA. The computer UT transmits a control signal, SDSS, which triggers the generation of a sensory stimulus by the device DSS; the response of the brain of the patient P to this stimulus is measured in the form of EEG signals, SEEG. The signals SDSS and SEEG are acquired by respective measurement channels of the system SA and are transmitted to the computer UT. Since the propagation delays of the signals are known, or in any event constant, the latter can easily place them on a common time base.
The document WO 01/06922 describes a system of this type. In this system, a stimulation device (a video recorder) and an EEG signal analog-digital converter are simultaneously activated by means of respective electronic switches; a computer then acquires the video signals at the output of the video recorder and also the digitized EEG signals.
This approach cannot be transposed directly onto wireless acquisition systems, or, if so, only with very imprecise synchronization. In fact, wireless data transmission systems in most cases present a delay (“latency”) which is relatively substantial (in the order of several hundred milliseconds), which is unknown and—above all—variable. In fact, for reasons of robustness, wireless transmission protocols generally implement sophisticated error detection and correction algorithms which may entail repeated exchanges between the receiver and transmitter. The time required to transmit a data packet therefore depends on the quality of the link (signal-to-noise ratio, interferences, etc.), the transmission speed and also the workload of the computer connected to the receiver. Thus, in wireless communication systems, the latency is not controlled (it is unknown and variable in an unforeseeable manner). This situation is illustrated by FIG. 2, which shows an acquisition system comprising two separate acquisition systems: a first, SA1, intended for the acquisition of the signal SEEG and comprising a transmit terminal TE connected to a receive terminal TR via a wireless link CD; and a second, SA2, intended for the acquisition of the signal SDSS. The two acquisition systems do not communicate with one another (or, if so, only via a wireless link with unmanaged latency). Consequently, it is not possible to locate the signals SEEG and SDSS on a common time base, except by ignoring the latency of the wireless link (or assuming it to be constant), which entails substantial synchronization errors.